Compound semiconductor film, solar cell, and methods for producing those

ABSTRACT

A compound semiconductor film is formed with a compound containing: A. at least one element selected from zinc, tin, cadmium, indium, and gallium; B. at least one element selected from oxygen and sulfur; and C. an element of Group IIa. A solar cell is configured to include: a substrate ( 11 ); a conductive layer ( 12 ) formed on the substrate ( 11 ); a light-absorption layer ( 13 ) that is formed on the conductive layer ( 12 ) with a compound semiconductor containing an element of Group Ib, an element of Group IIIa, and an element of Group VIa; the above-described compound semiconductor film ( 14 ) formed on the light-absorption layer ( 13 ); and a transparent conductive layer ( 16 ) formed on the compound semiconductor film ( 14 ). Such a configuration provides a compound semiconductor film having a low electric resistivity. Further by employing the compound semiconductor film having a low electric resistivity as a buffer layer of a solar cell, the energy conversion efficiency of the solar cell is improved.

TECHNICAL FIELD

The present invention relates to a compound semiconductor film,particularly zinc-based, tin-based, cadmium-based, indium-based, orgallium-based compound semiconductor film. The present invention alsorelates to a solar cell, particularly a solar cell provided with alight-absorption layer formed with a compound semiconductor containingan element of Group Ib, an element of Group IIIb, and an element ofGroup VIb.

BACKGROUND ART

A thin film solar cell using CuInSe₂ (hereinafter referred to as CIS) orCu(In, Ga)Se₂ (hereinafter referred to as CIGS) as a light-absorptionlayer has an advantage of showing a high energy conversion efficiencyand being free from a deterioration in the energy conversion efficiencycaused by the irradiation with light or the like, where the CuInSe₂ is acompound semiconductor (chalcopyrite structured compound semiconductor)containing an element of Group Ib, an element of Group IIIB and anelement of Group VIB, while the Cu(In, Ga)Se₂ is a solid solution of CISwith Ga.

A conventional CIS solar cell is formed by chemical deposition of ann-type semiconductor layer on a CIS film, and likewise a conventionalCIGS solar cell is formed by chemical deposition of an n-typesemiconductor layer on a CIGS film.

A conventional typical solar cell provided with a CIS film or CIGS filmas a liglit-absorption layer includes a substrate, a back-side electrodestacked on the substrate, a p-type light-absorption layer, a bufferlayer, a window layer, a transparent conductive layer, and a p-sideelectrode in contact with the back-side electrode, and an n-sideelectrode in contact with the transparent conductive layer.

In a conventional solar cell, a CdS film mainly has been used as thebuffer layer. However, since Cd is toxic, a buffer layer not containingCd has been demanded. For this purpose, a Zn-based buffer layer formedby employing a Zn-based compound semiconductor film has been developed(for instance, see the following non-patent document 1).

Non-patent document 1: Jpn. Appl. Phys. Vol. 35 (1996) pp.4383-4388,Hitoshi KUSHIYA, “Application of Zn-Compound Buffer Layer forPolycrystalline CuInSe₂-Based Thin Film Solar Cells)

However, since the conventional non-Cd-based buffer layer formed byemploying the non-Cd-based compound semiconductor film has an extremelyhigh electric resistance, there has been a problem that the solar cellprovided with the non-Cd-based buffer layer has a low energy conversionefficiency.

DISCLOSURE OF INVENTION

To solve the above-described problem of the prior art, it is an objectof the present invention to provide a compound semiconductor film with alow electric resistivity, a solar cell, and methods for producing thesame.

A compound semiconductor film of the present invention contains:

A. at least one element selected from the group consisting of zinc, tin,cadmium, indium, and gallium;

B. at least one element selected from oxygen and sulfur; and

C. an element of Group IIa.

A method of the present invention for producing a compound semiconductorfilm includes the steps of:

preparing a material solution by dissolving in water a compoundcontaining at least one selected from zinc, tin, cadmium, indium, andgallium, a compound containing sulfur, and a Group IIa element compoundcontaining an element of Group IIa; and

bringing the material solution prepared at a predetermined temperatureinto contact with a substrate so that a compound semiconductor film isdeposited on the substrate, the compound semiconductor film containing:

A. at least one element selected from zinc, tin, cadmium, indium, andgallium;

B. at least one element selected from oxygen and sulfur; and

C. an element of Group IIa.

A solar cell of the present invention is a solar cell obtained by eitherstacking a substrate, a conductive film, a light-absorption layer, acompound semiconductor film, and a transparent conductive layer in thestated order, or stacking a substrate, a conductive film, a compoundsemiconductor film, a light-absorption layer, and a transparentconductive layer in the stated order,

wherein the compound semiconductor film contains:

A. at least one element selected from zinc, tin, cadmium, indium, andgallium;

B. at least one element selected from oxygen and sulfur; and

C. an element of Group IIa.

A method of the present invention for producing a solar cell includesthe step of either:

stacking a substrate, a conductive film, a light-absorption layer, acompound semiconductor film, and a transparent conductive layer in thestated order; or

stacking a substrate, a conductive film, a compound semiconductor film,a light-absorption layer, and a transparent conductive layer in thestated order,

wherein the compound semiconductor film is formed by:

preparing a material solution by dissolving in water a compoundcontaining at least one selected from the group consisting of zinc, tin,cadmium, indium, and gallium, and a compound containing sulfur, and aGroup IIa element compound containing an element of Group IIa; and

bringing the material solution prepared at a predetermined temperatureinto contact with at least the substrate so that a compoundsemiconductor film is deposited, the compound semiconductor filmcontaining:

A. at least one element selected from zinc, tin, cadmium, indium, andgallium;

B. at least one element selected from oxygen and sulfur; and

C. an element of Group IIa.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a configuration of a solarcell according to an example of the present invention.

FIG. 2 is a graph showing volume resistivities of compound semiconductorfilms according to Example 3 of the present invention.

FIG. 3 is a graph showing energy conversion efficiencies of solar cellsaccording to Example 3 of the present invention.

DESCRIPTION OF THE INVENTION

A compound semiconductor film according to the present inventioncontains at least one element selected from zinc, tin, cadmium, indium,and gallium, at least one element selected from oxygen and sulfur, andan element of Group IIa. In the present specification, each group iscalled by the name according to the short-form periodic table. It shouldbe noted that “Group IIa” refers to the “Group 2” according to thelong-form periodic table recommended by the International Union of Pureand Applied Chemistry (IUPAC).

A solar cell according to the present invention includes a conductivelayer; a transparent conductive layer; a light-absorption layer that isprovided between the conductive layer and the transparent conductivelayer, and that is made of a compound semiconductor containing anelement of Group Ib, an element of Group IIIb, and an element of GroupVIb; and a compound semiconductor film that is provided between thelight-absorption layer and the transparent conductive layer, and thatcontains at least one element selected from zinc, tin, cadmium, indium,and gallium, at least one element selected from oxygen and sulfur, andan element of Group IIa. It should be noted that “Group Ib”, “GroupIIIB”, and “Group VIb” refer to “Group 11”, “Group 13”, and “Group 16”according to the long-form periodic table recommended by the IUPAC,respectively. The solar cell of the present invention may be a solarcell of the substrate type or a solar cell of the superstrate type.

More specifically, the substrate-type solar cell according to thepresent invention includes: a substrate; a conductive layer formed onthe substrate; a light-absorption layer that is formed on the conductivelayer and that is made of a compound semiconductor containing an elementof Group Ib, an element of Group IIIb, and an element of Group VIb; acompound semiconductor film that is formed on the light-absorption layerand that contains at least one element selected from zinc, tin, cadmium,indium, and gallium, at least one element selected from oxygen andsulfur, and an element of Group IIa; and a transparent conductive layerformed on the compound semiconductor film.

The superstrate-type solar cell according to the present inventionincludes: a substrate: a transparent conductive layer formed on thesubstrate; a compound semiconductor film that is formed on thetransparent conductive layer and that contains at least one elementselected from zinc, tin, cadmium, indium, and gallium, at least oneelement selected from oxygen and sulfur, and an element of Group IIa; alight-absorption layer that is formed on the compound semiconductor filmand that is made of a compound semiconductor containing an element ofGroup Ib, an element of Group IIIb, and an element of Group VIb; and aconductive layer formed on the light-absorption layer.

According to the present invention, an element of Group IIa is doped inthe compound semiconductor film, whereby an electric resistance of thecompound semiconductor film can be reduced. Further, according to thepresent invention, the compound semiconductor film with a reducedelectric resistance is used as a buffer layer, whereby an energyconversion efficiency of the solar cell can be improved.

The compound semiconductor film of the present invention contains atleast one selected from Zn, Sn, Cd, In, and Ga, at least one selectedfrom O and S, and an element of Group IIa, as described above. Thecompound semiconductor film of the present invention may be configuredso that, regarding the element of Group IIa contained therein, oneelement of Group IIa may be contained, or a plurality of differentelements of Group IIa may be contained. The elements of Group IIa to becontained in the compound semiconductor film preferably are Mg, Ca, Sr,and Ba. By adding such elements, an electric resistance of the compoundsemiconductor film can be reduced excellently.

The method for producing the compound semiconductor film of the presentinvention, as described above, is a method that employs a materialsolution (aqueous solution) obtained by dissolving in water a compoundcontaining at least one element selected from zinc, tin, cadmium,indium, and gallium, a sulfur containing compound that contains sulfur,and a Group IIa element compound that contains an element of Group IIa,so that a compound semiconductor film is grown in the liquid phase on asubstrate. By this producing method, a compound semiconductor filmcontaining at least one selected from Zn, Sn, Cd, In, and Ga, at leastone selected from O and S, and an element of Group IIa can be produced.The material solution may be a solution in which ammonia, an ammoniumsalt, etc., is dissolved additionally.

As the Group IIa element compound, for instance, any one selected fromthe following is dissolved in the material solution: one compoundcontaining one element of Group IIa; a plurality of compounds each ofwhich contains the same one element of Group IIa; one compoundcontaining a plurality of elements of Group IIa; and a plurality ofcompounds containing a plurality of elements of Group IIa. Further, asthe compound containing at least one element selected from zinc, tin,cadmium, indium, and gallium, for example, any one selected from thefollowing is dissolved in the material solution: one compound containingat least one element selected from zinc, tin, cadmium, indium, andgallium; and a plurality of compounds each of which contains at leastone element selected from zinc, tin, cadmium, indium, and gallium. Stillfurther, as the sulfur containing compound, for instance, either onecompound containing sulfur, or a plurality of compounds each of whichcontains sulfur, are dissolved in the material solution.

In the method of the present invention for producing a compoundsemiconductor film, as the Group IIa element compound, the following canbe used: a chloride of an element of Group IIa; an iodide of an elementof Group IIa; a bromide of an element of Group IIa; a nitrate of anelement of Group IIa; a sulfate of an element of Group IIa; or anacetate of an element of Group IIa.

In the method of the present invention for producing a compoundsemiconductor film, the material solution preferably has a pH in a rangeof not less than 9 and not more than 11 in the step in which a compoundis deposited. In the case where the pH of the material solution is inthe foregoing range, a compound semiconductor film with high quality canbe produced. It should be noted that the pH of the material solutionvaries with the temperature of the material solution.

The pH of the material solution can be adjusted by, for instance,dissolving ammonia and/or an ammonium salt in the material solution.Further, by dissolving ammonia and/or an ammonium salt in the materialsolution, Zn, Cd, etc. is caused to form a complex. Zn, Cd, etc. isstabilized by forming a complex, whereby the film formation reactionproceeds slowly. Consequently a dense and high-quality compound can beformed. Accordingly, in the step of preparing the material solution, thepH of the material solution preferably is adjusted by dissolving ammoniaadditionally in water. This is because by using the material solution inwhich ammonia is dissolved, the compound semiconductor film of thepresent invention can be produced efficiently. Further, in the step ofpreparing the material solution, the pH of the material solutionpreferably is adjusted by dissolving an ammonium salt additionally inwater. This is because by using the material solution in which ammoniaand an ammonium salt are dissolved, the compound semiconductor film ofthe present invention can be produced efficiently without a groundworklayer (substrate) of the compound semiconductor film being damaged. Inthe method of the present invention for producing a compoundsemiconductor film, as the ammonium salt, for instance, at least onecompound selected from the group consisting of ammonium acetate,ammonium chloride, ammonium iodide, and ammonium sulfate can be used.

In the method of the present invention for producing a compoundsemiconductor film, the predetermined temperature of the materialsolution in the step in which a compound is deposited is preferably in arange of not lower than 10° C. and not higher than 100° C. This isbecause in the case where the temperature of the material solution is inthe foregoing range, a high-quality compound semiconductor film can beproduced.

When the material solution and the substrate are brought into contactwith each other, the material solution may be applied over a surface ofthe substrate, or alternatively, only a part of the substrate includinga part of a surface of the substrate may be brought into contact withthe material solution, or further alternatively, the substrate may beimmersed in the material solution. In the method of the presentinvention for producing a compound semiconductor film, the substratepreferably is immersed in the material solution in the step ofdepositing a compound. This is because a homogeneous compoundsemiconductor film can be produced by using the immersion method, andfurther, a film thickness of the compound semiconductor film can becontrolled simply by controlling the immersion time.

In the method of the present invention for producing a compoundsemiconductor film, in the step of preparing the material solution, atleast one compound selected from the group consisting of acetates,chlorides, iodides, and sulfates can be dissolved as the compound inwater.

In the method of the present invention for producing a compoundsemiconductor film, at least one compound selected from the groupconsisting of thiourea and thioacetamide can be dissolved as a compoundcontaining sulfur in water in the step of preparing the materialsolution.

The solar cell of the present invention is, as described above,configured so that the compound semiconductor film of the presentinvention is provided between a light-absorption layer and a transparentconductive layer. It should be noted that the compound semiconductorfilm of the present invention functions as a buffer layer in the solarcell. This configuration reduces an electric resistance of the bufferlayer, whereby the energy conversion efficiency can be improved.

The solar cell of the present invention is, as described above,configured so that the compound semiconductor film of the presentinvention is provided as a buffer layer. The compound semiconductor filmof the present invention has a small electric resistance as comparedwith a conventional compound semiconductor film that does not contain anelement of Group IIa. Therefore, the energy conversion efficiency of thesolar cell can be improved.

The solar cell of the present invention preferably is configured so thatthe element of Group IIa used therein is magnesium, calcium, strontium,or barium.

Further, the general formula of the compound semiconductor film of thepresent invention preferably is MIIa_(x)(O, S) or MIIa_(x)(O, OH, S)(where M represents at least one element selected from zinc, tin,cadmium, indium, and gallium, and x represents a value in a range of0.0008 to 0.012). In the case where the content of the element of GroupIIa is in the foregoing range, the compound semiconductor film functionsexcellently as a buffer layer of the solar cell.

The solar cell of the present invention can be configured so that acompound semiconductor in the light-absorption layer contains Cu as anelement of Group Ib, at least one element selected from the groupconsisting of In and Ga as an element of Group IIIb, and at least oneelement selected from the group consisting of Se and S as an element ofGroup VIb.

In the method of the present invention for producing a solar cell, asdescribed above, the method of the present invention for producing acompound semiconductor film is used for forming the compoundsemiconductor film that functions as a buffer layer. It should be notedthat in the production of the solar cell of the present invention, anyknown technologies may be used for steps other than the step of forminga compound semiconductor film.

According to the method of the present invention for producing a solarcell, a compound semiconductor film having a desired electric resistancecan be formed by controlling the content of an element of Group IIa inthe compound semiconductor film and the thickness of the compoundsemiconductor film. The content of an element of Group IIa in thecompound semiconductor film can be controlled by controlling theconcentration of the Group IIa element compound dissolved in thematerial solution and the temperature of the material solution in thedeposition of the compound.

EXAMPLE 1

The following describes, as Example 1, a case where a substrate having aCuInSe₂ thin film on its surface and a material solution are broughtinto contact with each other so that a Zn-based compound semiconductorfilm is formed on the CuInSe₂ thin film of the substrate.

First, a Mo film was formed by sputtering on a glass substrate.Thereafter, a CuInSe₂ thin film was formed on the Mo film. With this,the preparation of the substrate was completed.

Next, a material solution for forming a Zn-based compound semiconductorfilm was prepared by dissolving zinc chloride (ZnCl₂) as a Zn compound(salt) containing zinc, thiourea (NH₂CSNH₂) as a compound containingsulfur, calcium chloride (CaCl₂) as a Group IIa element compoundcontaining Ca, and ammonium chloride (NH₄Cl) in water. The materialsolution was prepared so that the concentration of zinc chloride was0.01 mol/L, the concentration of thiourea was 0.2 mol/L, theconcentration of calcium chloride was 0.0001 mol/L, and theconcentration of ammonium chloride was 0.5 mol/L. After preparing thematerial solution, a container containing the material solution was leftto stand in a hot water vessel whose temperature was kept at 85° C., sothat the temperature of the material solution was adjusted to 85° C.After the temperature of the material solution was stabilized, thesubstrate having a CuInSe₂ thin film was immersed in the materialsolution for about 20 minutes. Thereafter, the substrate was removed outof the material solution, and subsequently was washed with pure water.Through this process, a Zn-based compound semiconductor film wasobtained.

The composition of the Zn-based compound semiconductor film of thepresent Example 1 was analyzed by X-ray photoelectron spectroscopy. Inthis analysis, Zn, O, and S were detected in the Zn-based compoundsemiconductor film. Further, by determining a bond energy of O on the 1sorbit, it was clarified that O in the Zn-based compound semiconductorfilm was present in two states, i.e., a state of O alone, and a state ofOH. Further, analysis by an inductively coupled plasma (ICP)spectrometry device showed that Ca was contained in the Zn-basedcompound semiconductor film. The concentrations of the elements were asfollows: the concentration of Zn was 46.24 atom %, the concentration ofO was 37.8 atom %, the concentration of S was 11.1 atom %, theconcentration of H was 4.4 atom %, and the concentration of Ca wasapproximately 0.46 atom %.

According to the foregoing producing method, a Zn(O, OH, S):Ca film wasformed easily on the CuInSe₂ thin film of the substrate.

The obtained compound semiconductor film had a volume resistivity (Ω·cm)of 2×10⁹.

Next, an example of a solar cell provided with the foregoing Zn-basedcompound semiconductor film as a buffer layer is described in thefollowing, with reference to FIG. 1.

The solar cell shown in FIG. 1 is a substrate-type solar cell thatincludes: a glass substrate 11 (substrate); a Mo film 12 (conductivelayer) formed on the glass substrate 11; a Cu(In, Ga)Se₂ film 13(light-absorption layer) formed on a surface of a part of the Mo film12; a p-side lead electrode 17 formed on a surface of another part ofthe Mo film 12; a Zn(O, OH, S):Ca film 14 (Zn-based compoundsemiconductor film) formed on the Cu(In, Ga)Se₂ film 13; a ZnO film 15(window layer) formed on the Zn(O, OH, S):Ca film 14; an indium-tinoxide alloy (ITO) film 16 (transparent conductive layer) formed on theZnO film 15; and a n-side lead electrode 18 formed on a surface of apart of the ITO film 16.

The solar cell shown in FIG. 1 was produced through the followingprocess. First, the glass substrate 11 was prepared. Next, the Mo film12 (film thickness: 1 μm) was formed by sputtering as a conductive layer(backside electrode) over an entirety of the glass substrate 11. Then,the Cu(In, Ga)Se₂ film 13 (film thickness: 2 μm) was formed by vapordeposition over an entirety of the Mo film 12.

Next, the Zn(O, OH, S):Ca film 14 (film thickness: 100 nm) was formed bythe same method as above over an entirety of the Cu(In, Ga)Se₂ film 13.Subsequently, the ZnO film 15 (film thickness: 100 nm) was formed bysputtering over an entirety of the Zn(O, OH, S):Ca film 14. Then, theITO film 16 (film thickness: 100 nm) was formed by sputtering over anentirety of the ZnO film 15. The sputtering for forming the ZnO film 15and the ITO film 16 was carried out in an argon gas atmosphere at a gaspressure of 1.07 Pa (8×10⁻³ Torr), with a high-frequency power of 500 Wbeing applied to a target. Then, after the ITO film 16 was formed, apart of the Mo film 12 was exposed by mechanically removing the Cu(In,Ga)Se₂ film by taking an advantage of the hardness difference between Moand Cu(In, Ga)Se₂. Next, a mask on which an electrode pattern was drawnwas attached so as to cover the surface of the ITO film 16 and theexposed surface of the Mo film 12, and a NiCr film was formed byelectron beam evaporation, and successively an Au film was formed byelectron beam evaporation over an entirety of the NiCr film, so that thep-side lead electrode 17 (film thickness: 350 nm) and the n-sideelectrode 18 (film thickness: 350 nm) were formed. As a result, thesolar cell shown in FIG. 1 was completed.

A pseudo-solar light beam with an AM (Air Mass) of 1.5 and an intensityof 100 mW/cm² was projected onto the solar cell shown in FIG. 1, andsolar cell properties were assessed. Consequently, the solar cellexhibited an open circuit voltage of 0.64 V, a short circuit current of34.2 mA/cm², a fill factor of 0.65, and an energy conversion efficiencyof 14.2%.

COMPARATIVE EXAMPLE 1

For comparison with Example 1, a solar cell as Comparative Example wasproduced by the same method as that for producing the solar cell shownin FIG. 1 except that a Zn(O, OH, S) film was formed instead of theZn(O, OH, S):Ca film 14 in Example 1. It should be noted that the Zn(O,OH, S) film was formed by the same method as that for forming the Zn(O,OH, S):Ca film 14 except that calcium chloride was not dissolved in thematerial solution.

Regarding the solar cell of Comparative Example also, solar cellproperties were assessed by the same method as that for assessing thesolar cell shown in FIG. 1. Consequently, the solar cell of ComparativeExample exhibited an open circuit voltage of 0.26 V, a short circuitcurrent of 12.0 mA/cm², a fill factor of 0.23, and an energy conversionefficiency of 0.7%.

The solar cell properties of the solar cell of Comparative Example wereinferior to those of the solar cell shown in FIG. 1. This inferiority isdue to a high volume resistivity of the Zn(O, OH, S) film.

EXAMPLE 2

The following describes, as Example 2, a case where a substrate having aCu(In, Ga)Se₂ thin film on its surface and a material solution arebrought into contact with each other so that a Zn-based compoundsemiconductor film is formed on the Cu(In, Ga)Se₂ thin film of thesubstrate.

First, a Mo film was formed by sputtering on a glass substrate.Thereafter, a Cu(In, Ga)Se₂ thin film was formed on the Mo film. Withthis, the preparation of the substrate was completed.

Next, a material solution was prepared by dissolving zinc acetate(Zn(CH₃COO)₂) as a Zn compound (salt) containing zinc, thiourea(NH₂CSNH₂) as a S compound containing sulfur, barium chloride (BaCl₂) asa Group IIa element compound containing Ba, ammonia (NH₃), and ammoniumacetate (CH₃COONH₄) in water. The material solution was prepared so thatthe concentration of zinc acetate was 0.01 mol/L, the concentration ofthiourea was 0.2 mol/L, the concentration of barium chloride was 0.0001mol/L, the concentration of ammonia was 0.5 mol/L, and the concentrationof ammonium acetate was 0.1 mol/L. After preparing the materialsolution, a container containing the material solution was left to standin a hot water vessel whose temperature was kept at 85° C., so that thetemperature of the material solution was adjusted to 85° C. After thetemperature of the material solution was stabilized, the substratehaving a Cu(In, Ga)Se₂ thin film was immersed in the material solutionfor about 20 minutes. Thereafter, the substrate was removed out of thematerial solution, and subsequently was washed with pure water. Throughthis process, the formation of a Zn-based compound semiconductor filmwas completed.

The composition of the Zn-based compound semiconductor film of thepresent Example 2 was analyzed by X-ray photoelectron spectroscopy. Inthis analysis, Zn, O and S were detected in the Zn-based compoundsemiconductor film. Further, by determining a bond energy of O on the 1sorbit, it was clarified that O in the Zn-based compound semiconductorfilm was present in two states, i.e., a state of O alone, and a state ofOH. Further, analysis by an inductively coupled plasma (ICP)spectrometry device showed that Ba was contained in the Zn-basedcompound semiconductor film. The concentrations of the elements were asfollows: the concentration of Zn was 46.24 atom %, the concentration ofO was 37.8 atom %, the concentration of S was 11.1 atom %, theconcentration of H was 4.4 atom %, and the concentration of Ba was 0.46atom %. In other words, Ba accounted for 0.01 atom % per one atom of Zn.

According to the foregoing producing method, a Zn(O, OH, S):Ba film wasformed easily on the Cu(In, Ga)Se₂ thin film.

The obtained compound semiconductor film had a volume resistivity (Ω·cm)of 2×10⁹.

Next, a solar cell was formed in the same manner as that of Example 1. Apseudo-solar light beam with an AM (Air Mass) of 1.5 and an intensity of100 mW/cm² was projected to the solar cell, and solar cell propertieswere assessed. Consequently, the open circuit voltage was 0.65 V, theshort circuit current was 34.5 mA/cm², the fill factor was 0.66, and theenergy conversion efficiency was 14.8%.

As Examples 1 and 2, examples in which Zn(O, OH, S)_(x):Group IIaelement was used were described, but the same effect can be achieved byusing Zn(O, S)_(x):Group IIa element, Sn(O, OH, S)_(x):Group IIaelement, Cd(O, OH, S)_(x):Group IIa element, CdZn(O, OH, S)_(x):GroupIIa element, ZnSn (O, OH, S)_(x):Group IIa element, In(O, OH,S)_(x):Group IIa element, Ga(O, OH, S)_(x):Group IIa element, InGa(O,OH, S)_(x):Group IIa element, ZnGa(O, OH, S)_(x):Group IIa element,ZnIn(O, OH, S)_(x):Group IIa element, CdS:Group IIa element, ZnS:GroupIIa element, In₂S₃:Group IIa element, Ga₂S₃:Group IIa element,In₂O₃:Group IIa element, or Ga₂O₃:Group IIa element.

EXAMPLE 3

Experiments were carried out in the same manner as the foregoingexamples except that an amount of calcium chloride (CaCl₂) added wasvaried and the value of x of ZnCa_(x)(O, OH, S) was set as shown inTable 1 below. Values regarding compound semiconductor films and solarcells detected as a result of the experiments are shown in Table 1.TABLE 1 Open Short Energy Value of x of Volume Circuit CircuitConversion Experiment ZnCa_(x)(O, OH, S) Resistivity Voltage CurrentFill Efficiency No. (atomic ratio) (Ω · cm) (V) (mA/cm²) Factor (%) 1 — 1 × 10¹³ 0.26 8.5 0.23 0.5 2 0.0001  1 × 10¹³ 0.26 12.0 0.33 1.0 30.0008 8 × 10⁹ 0.63 32.8 0.63 13.0 4 0.001 8 × 10⁹ 0.63 33.1 0.63 13.1 50.005 4 × 10⁹ 0.63 33.6 0.64 13.5 6 0.01 2 × 10⁹ 0.64 34.2 0.65 14.2 70.012 2 × 10⁹ 0.64 34.1 0.65 14.2 8 0.1 4 × 10⁸ 0.45 26.0 0.43 5.0

As is clear from Table 1, the compound semiconductor films and the solarcells of the present example exhibited excellent outcomes.

The detected volume resistivities of the compound semiconductor films ofExample 3 are shown in the graph of FIG. 2. The graph shows that,particularly preferably, the value of x of ZnCa_(x)(O, OH, S) is in arange of 0.0008 to 0.012 (atomic ratio).

FIG. 3 is a graph of energy conversion efficiencies (%) of solar cellsof Example 3. The graph shows that the value of x of ZnCa_(x)(O, OH, S)is particularly preferably in a range of 0.0008 to 0.012 (atomic ratio).

EXAMPLE 4

Experiments were carried out in the same manner as Example 1 except thatmagnesium chloride (MgCl₂) or strontium chloride (SrCl₂) was used inplace of calcium chloride (CaCl₂). Values regarding compoundsemiconductor films and solar cells detected as a result of theexperiments are shown in Table 2. Here, in the row of “the added amountof the Group IIa element compound (atomic ratio)”, general formulae ofthe compound semiconductor films and values of x of MIIa_(x)(O, OH, S)(where M represents Zn) are shown. TABLE 2 Added Amount of Group IIaOpen Short Energy Element Volume Circuit Circuit Conversion Experiment.Compound Resistivity Voltage Current Fill Efficiency No. (atomic ratio)(Ω · cm) (V) (mA/cm²) Factor (%) 9 MgCl₂ 9 × 10⁹ 0.63 34.8 0.66 14.50.001 10 MgCl₂ 2 × 10⁹ 0.62 33.6 0.64 13.3 0.01 11 SrCl₂ 7 × 10⁹ 0.6334.5 0.65 14.1 0.001 12 SrCl₂ 3 × 10⁹ 0.63 33.8 0.63 13.4 0.01

As is clear from Table 2, the compound semiconductor films and the solarcells of the present example exhibited excellent outcomes.

EXAMPLE 5

Experiments were carried out by altering the component containing themetal element in Example 1. The experiments were carried out in the samemanner as that for Example 1 except that tin chloride, cadmium chlorideindium chloride, or gallium chloride was used in place of zinc chloride(ZnCl₂). Values regarding compound semiconductor films and solar cellsdetected as a result of the experiments are shown in Table 3. TABLE 3Added Amount of Group IIa Open Short Energy Element Volume CircuitCircuit Conversion Experiment Compound Resistivity Voltage Current FillEfficiency No. (atomic ratio) (Ω ·cm) (V) (mA/cm²) Factor (%) 13 tinchloride 6 × 10⁹ 0.64 34.5 0.63 13.9 0.005 14 cadmium 4 × 10⁹ 0.63 34.00.64 13.7 chloride 0.005 15 indium 6 × 10⁹ 0.65 33.1 0.64 13.8 chloride0.005 16 gallium 5 × 10⁹ 0.65 34.2 0.64 14.2 chloride 0.005

As is clear from Table 3, the compound semiconductor films and the solarcells of the present example exhibited excellent outcomes.

INDUSTRIAL APPLICABILITY

The present invention can be utilized for improving properties of asolar cell, particularly its energy conversion efficiency.

1. A compound semiconductor film being a film of a semiconductorcontaining: A. at least one element selected from zinc, tin, cadmium,indium, and gallium; B. at least one element selected from oxygen andsulfur; and C. an element of Group IIa.
 2. The compound semiconductorfilm according to claim 1, wherein the element of Group IIa is at leastone element selected from magnesium, calcium, strontium, and barium. 3.The compound semiconductor film according to claim 1, wherein thecompound semiconductor is: Zn(O, S):element of Group IIa; Zn(O, OH,S):element of Group IIa; Sn(O, OH, S):element of Group IIa; Cd(O, OH,S):element of Group IIa; CdZn(O, OH, S):element of Group IIa; ZnSn (O,OH, S):element of Group IIa; In(O, OH, S):element of Group IIa Ga(O, OH,S):element of Group IIa; InGa(O, OH, S):element of Group IIa ZnGa(O, OH,S):element of Group IIa ZnIn(O, OH, S):element of Group IIa; CdS:elementof Group IIa; ZnS:element of Group IIa; In₂S₃:element of Group IIa;Ga₂S₃:element of Group IIa; In₂O₃:element of Group IIa; or Ga₂O₃:elementof Group IIa, where bracketed symbols of the elements represent aniongroups necessary for keeping charge neutrality with metal ions (iongroups), the charge neutrality being kept by metal ions (ion groups) andanions in the brackets.
 4. The compound semiconductor film according toclaim 1, having a volume resistivity of not less than 5×10⁸Ω·cm and notmore than 1×10¹¹ Ω·cm.
 5. The compound semiconductor film according toclaim 1, being expressed by a general formula of MIIa_(x)(O, S) orMIIa_(x)(O, OH, S), where M represents at least one element selectedfrom zinc, tin, cadmium, indium, and gallium, and x represents a valuein a range of 0.0008 to 0.012.
 6. The compound semiconductor filmaccording to claim 1, having a film thickness in a range of not lessthan 10 nm and not more than 150 nm.
 7. A method for producing acompound semiconductor film comprising the steps of preparing a materialsolution by dissolving in water a compound containing at least oneselected from the group consisting of zinc, tin, cadmium, indium, andgallium, and a compound containing sulfur, and a Group IIa elementcompound containing an element of Group IIa; and bringing the materialsolution prepared at a predetermined temperature into contact with asubstrate so that a compound semiconductor film is deposited on thesubstrate, the compound semiconductor film containing: A. at least oneelement selected from zinc, tin, cadmium, indium, and gallium; B. atleast one element selected from oxygen and sulfur; and C. an element ofGroup IIa.
 8. The method for producing a compound semiconductor filmaccording to claim 7, wherein as the Group IIa element compound, achloride of an element of Group IIa, an iodide of an element of GroupIIa, a bromide of an element of Group IIa, a nitrate of an element ofGroup IIa, a sulfate of an element of Group IIa, or an acetate of anelement of Group IIa is used.
 9. The method for producing a compoundsemiconductor film according to claim 7, wherein in the step in whichthe compound semiconductor film is deposited, the material solution hasa pH in a range of not less than 9 to not more than
 11. 10. The methodfor producing a compound semiconductor film according to claim 7,wherein in the step of preparing the material solution, the pH of thematerial solution is adjusted by dissolving ammonia additionally in thewater.
 11. The method for producing a compound semiconductor filmaccording to claim 7, wherein in the step of preparing the materialsolution, the pH of the material solution is adjusted by dissolving anammonium salt additionally in the water.
 12. The method for producing acompound semiconductor film according to claim 11, wherein in the stepof preparing the material solution, at least one compound selected fromthe group consisting of ammonium acetate, ammonium chloride, ammoniumiodide, and ammonium sulfate is used as the ammonium salt.
 13. Themethod for producing a compound semiconductor film according to claim 7,wherein in the step in which the compound semiconductor film isdeposited, the predetermined temperature of the material solution is ina range of not lower than 10° C. and not higher than 100° C.
 14. Themethod for producing a compound semiconductor film according to claim 7,wherein in the step in which the compound semiconductor film isdeposited, the substrate is immersed in the material solution.
 15. Themethod for producing a compound semiconductor film according to claim 7,wherein in the step of preparing the material solution, at least onecompound selected from the group consisting of acetates, chlorides,iodides, and sulfates is dissolved in the water as the compoundcontaining a metal.
 16. The method for producing a compoundsemiconductor film according to claim 7, wherein in the step ofpreparing the material solution, at least one compound selected from thegroup consisting of thiourea and thioacetamide is dissolved in the wateras the compound containing sulfur.
 17. A solar cell comprising. asubstrate; a conductive film; a light-absorption layer; a compoundsemiconductor film; and a transparent conductive layer, these beingstacked in the stated order, or in the order of the substrate, theconductive film, the compound semiconductor film, the light-absorptionlayer, and the transparent conductive layer, wherein the compoundsemiconductor film contains: A. at least one element selected from zinc,tin, cadmium, indium, and gallium; B. at least one element selected fromoxygen and sulfur; and C. an element of Group IIa.
 18. The solar cellaccording to claim 17, wherein the element of Group IIa is magnesium,calcium, strontium, or barium.
 19. The solar cell according to claim 17,wherein the compound semiconductor film is expressed by a generalformula of MIIa_(x)(O, S) or MIIa_(x)(O, OH, S), where M represents atleast one element selected from zinc, tin, cadmium, indium, and gallium,and x represents a value in a range of 0.0008 to 0.012.
 20. The solarcell according to claim 17, wherein the compound semiconductor film hasa film thickness in a range of not less than 10 nm and not more than 150nm.
 21. The solar cell according to claim 17, wherein thelight-absorption layer is formed with a compound semiconductorcontaining an element of Group Ib, an element of Group IIIa, and anelement of Group VIa.
 22. The solar cell according to claim 17, whereinthe compound semiconductor of the light-absorption layer contains: Cu asthe element of Group Ib; at least one element selected from the groupconsisting of In and Ga as the element of Group IIIb; and at least oneelement selected from the group consisting of Se and S as the element ofGroup VIb.
 23. A method for producing a solar cell comprising the stepof either: stacking a substrate, a conductive film, a light-absorptionlayer, a compound semiconductor film, and a transparent conductive layerin the stated order; or stacking a substrate, a conductive film, acompound semiconductor film, a light-absorption layer, and a transparentconductive layer in the stated order, wherein the compound semiconductorfilm is formed by: preparing a material solution by dissolving in watera compound containing at least one selected from zinc, tin, cadmium,indium, and gallium, a compound containing sulfur, and a Group IIaelement compound containing an element of Group IIa; and bringing thematerial solution prepared at a predetermined temperature into contactwith at least the substrate so that a compound semiconductor film isdeposited, the compound semiconductor film containing: A. at least oneelement selected from zinc, tin, cadmium, indium, and gallium; B. atleast one element selected from oxygen and sulfur; and C. an element ofGroup IIa.